Image forming system and apparatus with different printing modes for different numbers of printing sheets

ABSTRACT

An image forming system includes an image forming apparatus including a heating fixing portion and a host computer capable of instructing printing. In the image forming apparatus, throughput can be changed and discriminated in accordance with a printing number. For printing on small size sheets, the system is operable in a normal small size sheet mode and in a high speed small size sheet output mode, in which the printing is effected at a throughput which is higher than that in the normal small size sheet mode and, after completion of the printing, the image forming apparatus rests for a predetermined rest period. The host computer includes a mode selector for selecting a mode from the high speed small size sheet output mode and the normal small size sheet mode, and a controller for transmitting the mode selected by the mode selector to the image forming apparatus.

FIELD OF THE INVENTION

The present invention relates to an image forming system and an imageforming apparatus comprising host computer and an image formingapparatus, and more particularly to an improvement in a throughput ofsmall size sheet printing.

BACKGROUND ART

Conventionally, in a heat-fixing device (fixing device) provided in aimage forming apparatus employing a electrophotographic system or anelectrostatic recording system, a so-called heating roller type fixingdevice is widely used. In the fixing device, a recording materialcarrying the unfixed toner image is passed through a nip providedbetween a fixing roller and a pressing roller which are press-contactedto each other and are rotated by which the toner image is fixed on therecording material as a permanent image.

On the other hand, a film heating type fixing device has been put intopractice, in which no electric power is supplied to the fixing deviceduring a stand-by period, by which the electric energy consumption isminimized. Such a film heating type fixing device proposed and put topractical use as disclosed in Japanese Laid-open Patent Application Sho63-313182, Japanese Laid-open Patent Application Hei 2-157878, JapaneseLaid-open Patent Application Hei 4-44075 and Japanese Laid-open PatentApplication Hei 4-204980, for example.

FIG. 2 shows a typical film heating type fixing device. A fixing nip Nis formed by a heater 204, a pressing roller 202 supported by aheat-insulative holder 205 and a resin or metal fixing film 203 (fixingfilm) having a high heat conduction and sandwiched therebetween. Theunfixed toner image formed and carried on the recording material isintroduced into the fixing nip N and is heated and fixed. In order toprovide a sufficient width N of the fixing nip to form a satisfactoryfixed image, the fixing members including a heater 204 and a fixing film203 are urged to the pressing roller 202 by an urging spring 206 or thelike against the elastic of the pressing roller 202 In order to stablyprovide the fixing nip width N which is substantially uniform along thelongitudinal direction of the fixing member, a pressure substantiallyuniform along the longitudinal direction of the heat-insulative holder205 is applied through a metal stay 207 having a reverse U shape Inaddition, a structure in which a core metal at a end of the pressingroller is provided with a electroconductive rubber ring 209 such thatthe film potential is stabilized, is put into practice.

Recently, however, there are demands in an image forming apparatus suchas a copying machine or a printer, toward a high printing speed, quickstart, power save or downsizing. Because of the speed up of parts, thefixing temperature rises, and in order to accomplish the quick start,the improvement in the thermal responsivity of the heater and thereduction of the low thermal capacity thereof are intended. As a result,the temperature difference becomes large between the area in the fixingnip where the recording material exists (sheet passing area) and wherethe heat of the fixing device is deprived by the recording material andthe area where the recording material does not exist(non-sheet-passage-part) and where the heat is not deprived. Therefore,when a recording material (small size sheet) having a relatively smallwidth as compared with the length of the fixing device is fed into thefixing device, the temperature difference in the fixing device along thelongitudinal direction is large.

This means that a temperature difference between the proper fixingtemperature for the recording material and the destruction temperatureof the fixing device, that is, the margin is small. At present, in orderto reduce the temperature difference, as compared with the case in whicha relatively large recording material (full size sheet) is processed,when a small size sheet is processed, the printing speed is lowered(throughput down) to provide a time period for reducing the temperatureunevenness, in many examples. In the actual situations, limited numbersof sheets are processed randomly, but in conventional devices, thesetting of the throughput down is determined supposing that a largeamount of the small size sheets are continuously outputted. The resultis that for the actual use of the device, the margin against destructionis relatively large. Thus, in the case of outputting small size sheets,the throughput down as compared with the case of the large size sheetsis significantly large, which is not desirable for the users.

Prior art solving this problem proposes that a plurality of heatgenerating elements having different lengths are prepared, and thedifferent heat generating elements are used correspondingly to thedifferent lengths of the recording material. An example of such astructure is disclosed in Japanese Laid-open Patent Application2006-84805. However, with this structure, the problems of complicatedstructure of the device and the resulting cost increase arise, andtherefore, it is difficult to employ it in a low cost machine.

DISCLOSURE OF INVENTION

The present invention is made under the circumstances, and an objectthereof is to increase the small size sheet throughput at a low cost,thus improving the operability.

According to an aspect of the present invention, there is provided animage forming system comprising an image forming apparatus including aheating fixing portion and a host computer capable of instructingprinting to said image forming apparatus, wherein for the printing on asmall size sheet having a width smaller than a maximum processible widthof said image forming apparatus, said system is operable in a normalsmall size sheet mode, and in a high speed small size sheet output modein which the printing is effected at a throughput which is higher thanin the normal small size sheet mode, and after completion of theprinting, the image forming apparatus rests for a predetermined restperiod, wherein said host computer includes a mode selector forselecting a mode from the high speed small size sheet output mode andthe normal small size sheet mode, and a controller for transmitting themode selected by said mode selector to said image forming apparatus.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view schematically showing an image formingapparatus used with Embodiment 1.

FIG. 2 illustrates a structure of a heat-fixing device.

FIG. 3 is a block diagram schematically illustrating an image formingsystem in an Embodiment 1.

FIG. 4 shows an example of a setting screen for a small size paperprinting.

FIG. 5 is a flow chart showing processing in Embodiment 1.

FIG. 6 shows throughput comparison in Embodiment 1.

FIG. 7 shows results of end temperature raising experiments inEmbodiment 2.

FIG. 8 is a flow chart showing processing in Embodiment 2.

FIG. 9 is a flow chart showing processing in Embodiment 3.

FIG. 10 is a flow chart showing processing in Embodiment 4.

FIG. 11 is a flow chart showing processing in Embodiment 5.

FIG. 12 is a flow chart showing processing in Embodiment 6.

FIG. 13 is a schematic sectional view of a color image forming apparatusand a fixing device.

FIG. 14 is a graph showing a heat generation distribution of a fixingheater in Embodiment 7.

FIG. 15 is a graph and a Table showing average throughputs in acomparison example.

FIG. 16 is a flow chart showing processing in Embodiment 7.

FIG. 17 is a flow chart showing processing in a comparison example.

PREFERRED EMBODIMENTS OF THE INVENTION

An embodiment of the present invention in the form of an image formingsystem will be described in detail.

Embodiment 1

The image forming system according to Embodiment 1 will be described.

Referring to FIG. 1, the description will be made as to a laser beamprinter (LBP) which is an image forming apparatus used in the imageforming system according to this embodiment.

Here, the image forming apparatus is not limited to a LBP, but may be acopying machine, facsimile machine or the like.

FIG. 1 is a schematic sectional view illustrating a structure of theimage forming apparatus communicatable with an information processingapparatus.

In FIG. 1, designated by 101 is a main assembly of the LBP, whichreceives print data (including character codes, image data or the like),printing information comprising control codes, macro instructions or thelike which are supplied from a host computer or the like connected to anexternal device, and which stores them. And, it makes characterpatterns, form patterns or the like in accordance with the informationto form an image on a recording material.

Designated by 102 is an operation panel including switches for operationand LED displaying device or the like. Designated by 103 is a printingcontroller for controlling the main assembly 101 of the LBP and foranalyzing the letter information or the like supplied from the hostcomputer to effect the printing process. The printing information loadedin the printing controller 103 is converted to a pattern video signaland is supplied to a laser driver 104. The laser driver 104 includes acircuit for driving the semiconductor laser 105, and on-off-controls alaser beam L emitted by a semiconductor laser 105 in accordance withvideo signals inputted thereto. The laser beam L is deflected by arotatable polygonal mirror 106 in the left-light directions toscanningly expose the photosensitive drum 107 which has been uniformlycharged by a charging device 114.

By this, an electrostatic latent image corresponding to the imagepattern is formed on the photosensitive drum 107. The latent image isdeveloped and visualized by a developing device 108 provided adjacentthe photosensitive drum 107. As for the usable developing methods, thereare a jumping developing method, a two-component developing method, FEEDdeveloping method, and a combination of the image exposure and thereverse development is frequently used.

The visualized toner image is transferred from the photosensitive drum107 onto a recording material P fed at a predetermined timing, by atransfer roller 109 as a transferring device. In order to align theleading end of the image on the recording material with the imageformation position of the toner image on the photosensitive drum 107,the leading end of the recording material is detected by a top sensor110, and the timing is matched. The recording material P fed at thepredetermined timing is nipped and fed between the photosensitive drum107 and the transfer roller 109 at a constant pressure. The recordingmaterial P having the toner image transferred thereonto is fed to theheat-fixing device 111 and is fixed into a permanent image there.Residual toner remaining on the photosensitive drum 107 without beingtransferred is removed from the surface of the photosensitive drum 107by a cleaning device 112. Designated by 113 is a sheet discharge sensorprovided in the heat-fixing device 111, and functions to detect sheetjamming between the top sensor 110 and the sheet discharge sensor 113

FIG. 2 is a schematic illustration of the heat-fixing device (heatingfixing portion) 111 provided in the image forming apparatus. Theheat-fixing device 111 is a film heating type heat-fixing devicefundamentally comprising a fixing assembly 201 and a pressing roller 202which are press-contacted to each other to form a nip N.

As shown in sectional view (a) and perspective view (c) of FIG. 2, thefixing assembly 201 comprises mainly a fixing film 203, a heater 204, aheat-insulative holder 205 holding the heater 204, and a metal stay 207for receiving the pressure from the urging spring 206 to urge theheat-insulative holder 205 against the pressing roller 202.

As shown in (b) of FIG. 2, the heater 204 as the heating member iscontacted to the inner surface of the fixing film 203 to heat the nip N.The heater 204 is in the form of a plate and has a low thermal capacity,and comprises an insulative ceramic substrate 204 a of alumina, aluminumnitride or the like, and an electric heat generating resistance layer204 b of Ag/Pd (silver-palladium), RuO2, Ta2 N or the like providedthereon by screen printing or the like. The surface of the heater 204contacting the fixing film 203 is coated with a protection layer 204 cfor protecting the electric heat generating resistance layer which doesnot deteriorate the heat efficiency. The protection layer is preferablythin enough, and improves the surface property, and the material thereofis glass, fluorinated resin material or the like.

The heat-insulative holder 205 supporting the heater 204 is made of heatresistive resin material such as liquid crystal polymer, phenolic resin,PPS, PEEK or the like. The higher the thermal conductivity, the betterthe heat conduction to the pressing roller 202, and therefore, the resinmaterial layer may contain glass balloon, silica balloon or anotherfiller. The heat-insulative holder 205 functions also as a guide forrotation of the fixing film 203.

Designated by 207 is a metal stay and contacts the heat-insulativeholder 205 to suppress the flexion and/or twisting of the entirety ofthe fixing assembly.

In a temperature control of the heater 204, in accordance with thesignal from the temperature detecting element 208 such as a thermisterprovided on the rear surface of the ceramic substrate 204 a, an unshownCPU determines a duty ratio, the number waves or the like of a voltageapplied to the electric heat generating resistance layer to effect theproper control. By doing so, the temperature in the fixing nip is keptat a desired fixing set temperature.

Thus, a heat-fixing device shown in FIG. 2 comprising a heating element,a heat resistive film having one side slidingly contacting the heatingelement and the other side for contacting the recording material, apressing member in the form of a roller for driving the heat resistivefilm and for urging the recording material toward the heating elementthrough the heat resistive film. The heat resistive film and therecording material are nipped and fed together through the nip formed bythe heating element and the pressing member, during which the recordingmaterial is heated.

FIG. 3 is a block diagram showing a structure of the image formingsystem according to this embodiment, which comprises the host computerand the image forming apparatus (printing apparatus).

In FIG. 3, designated by 301 is a host computer and is effective tooutput the image data including print data or control code or the liketo the image forming apparatus 101.

It may be of a single unit type of multiple unit type which may beconnected wirelessly or non-wirelessly through a network such a LAN, aslong as the function of the present invention is accomplished.

From the standpoint of functions, the image forming apparatus 101generally comprises printing controller 311, an operation panel portion102, an output controller 313 and a printer engine portion 314.

The printing controller 311 comprise an interface (I/F) portion 310 as acommunicating portion with the host computer 301, a receive buffer 312for temporarily holding managing the received data, a sending buffer 315for holding temporarily and managing the sending data, a file system 316which is a storing portion for storing various data, for execution ofthe printing control, a data analyzing portion 317 for controllinganalysis of the print data, a printing control process executing portion318, image processor 319, and a page memory 320 or the like.

The interface (I/F) portion 310 functions as a communicating portion fortransaction of the print data with the host computer 301 and also as astate notifying portion for the state of the printer. The print datareceived through the interface (I/F) portion 310 and are stored in a 312which is a storing portion for temporarily storing the data, and readout and processed by the data analyzing portion 317 when necessary. Thedata analyzing portion 317 comprises a control program 321 correspondingto each printing control command. The command analyzed by data analyzingportion 317 converts the result of the analysis of the print datarelating to the imaging to universal intermediary codes which are easyto process by the image processor 319. The commands other than theimaging such as for sheet feed selection, form registration or the likeare processed in the printing control process executing portion 318. Inthe image processor 319, each imaging command is executed by the middlecode to load each object of the characters, the figures and the imagesinto the page memory 320 when necessary.

Generally, the controller 311 is a computer system using a centralprocessing unit (CPU), a read-only-memory (ROM), a random-access-memory(RAM) or the like. The processings of the each portion may be executedin time sharing under the control of a multi-task screen (real time,OS), or may be executed independently with preparation of an additionalcontroller hardware. The operation panel portion 102 functions to setand display various states of the image forming apparatus. The outputcontroller 313 converts the content of the page memory 320 to a videosignal to feed the image to the printer engine portion 314. The printerengine portion 314 is an image forming device for forming a permanentvisual image on the recording material on the basis of the receivedvideo signal, and has been in conjunction with FIG. 1.

The image forming apparatus 101 has been described, and the structure ofthe host computer 301 will be described, here.

The host computer 301 is a single computer system comprising keyboard303 which is an input device, a mouse 304 which is a pointing device, adisplay screen 302 which is a display device. The host computer 301 isoperated under the control of basic OS. Focusing the portion relating tothe printing alone, the function in the basic OS is divided into agraphic device interface (GDI) 306 which is a part of the functions ofapplication software 305 and the basic OS, and a printer spooler 308 fortemporarily storing the data generated by the printer driver 307 and theprinter driver.

Generally, in the host computer 301, the hardware such as the centralprocessing unit (CPU), the read-only-memory (ROM), therandom-access-memory (RAM) and the like is controlled by basic softwareto operate application software which is under the control of the basicsoftware. The printer driver 307, the printer spooler 308 or the like isone of the application software. By the application software 305,various data editing operations for the texts, the figures and theimages can be executed, and when the data are to be printed, an unshownprint instructing portion is selected by the mouse 304 or the like toexecute the printing.

Then, the application software 305 calls GDI 306 which is a function ofa part of the basic OS. The GDI 306 is a group of basic functions forcontrolling the display device such as the screen display on the displayscreen 302, the print output or the like, and the printing device.Various application software use the basic functions to execute theoperations irrespective of difference of the equipment (hardware).

In the GDI306, the information on the imaging performance or printresolution or the like of the printing device is fetched from theprinter driver 307 which controls the information depending on theactual equipment of the image forming apparatus, and the GDI functioncalled from application software 305 is analyzed, and

The information is supplied to the printer driver 307 currently selectedThe printer driver 307 generates the print data adapted to the functionsof the used printing apparatus on the basis of information received fromthe GDI306, and the printing ambient condition setting set by thegraphical user interface (GUI) possessed by itself or set by thecharacter user interface (CUI).

The generated print data may be a group of commands when the imageforming apparatus is capable of understanding the printer language(PDL), the image data when the image forming apparatus side effects onlythe image process, or all the data corresponding to the functions andpower of the image forming apparatus.

The print data thus generated are stored temporarily by the data storingportion called printer spooler 308. The printer spooler 308 is effectiveto release the application software from the printing process quickly.

That is, if the print data is sent directly to the image formingapparatus, the reaching to the capacity of the receiving buffer 312 ofthe image forming apparatus or the occurrence of off-line state of thecommunicating portion for one reason or another (sheet jamming, forexample) prevents the host computer 301 from sending the print data,with the result of interruption of the printing process of theapplication software.

By the provision of the means for temporarily storing the data, uponsending all of the print data into the storing portion, the applicationsoftware is released from the required printing process operation.

The print data thus generated is temporarily accumulated by the datastoring portion, that is, the printer spooler 308, and thereafter, isdelivered to the image forming apparatus 101 through the I/F portion 309which is the communicating portion of the host computer 301. The I/Fportion 309 functions also to receive the printing information from theimage forming apparatus 101.

The description has been made as to various elements relating to thisembodiment, and then the overall operations will be described.

With respect to this embodiment, a basic example of an execution of aprinting mode execution for a small size sheet using the host computer301 will be described. For the document edited and made by the user onthe application software (application) 305, the user effects theprinting instructions, and then the application software 305 calls theGDI 306 which is a part of the functions of the basic OS. The GDI 306,fetches the information of the imaging performance of the image formingapparatus, the print resolution and the like from the printer driver 307managing the information dependent on the current equipment of the imageforming apparatus, and analyzes the GDI function called from applicationsoftware 305, and expands the document data (information) into bit mapdata, and sends the data to the currently selected printer driver 307 asimage data.

The printer driver 307 receives the document data received from theGDI306 and the printing setting information set by the graphical userinterface (GUI) of the printer driver 307.

FIG. 4 illustrates an example of a print setting screen displayed on thedisplay screen 302, that is, the GUI screen of the printer driver 307 inthis embodiment. The user selects the mode on the screen.

The image forming apparatus of this embodiment is operable for printingof a small size sheet having a width smaller than a maximum operablewidth of the image forming apparatus, in a normal mode (first small sizepaper printing mode) and in a high speed output mode (second small sizepaper printing mode). Upon printing for a small size sheet, the normalmode and the high speed output mode are selectable by the GUI of theprinter driver 307. When the high speed output mode is selected, thethroughput is higher than in the normal mode, and after the end of theprinting, a rest period for a predetermined length is executed, as afeature of this embodiment. The image forming apparatus is operable, forprinting on the recording material having a small size which is narrowerthe maximum operable width of the image forming apparatus, the firstsmall size paper printing mode and a second small size paper printingmode in which the output number per unit time is larger than in thefirst small size paper printing mode with limited continuouslyoutputtable number.

FIG. 5 is a flow chart of data processing. Table 1 shows an example ofsetting of the high speed output mode in this embodiment, and FIG. 6shows each throughput.

TABLE 1 Example of high speed output mode setting Rest periodThroughputs after printing Normal small 18 ppm-14 ppm-9 ppm No sizesheet mode High speed 22 ppm(constant at 10 sec output mode for fullspeed) 5 or less sheets High speed 18 ppm (constant) 15 sec output modefor 10 or less than 10 sheets

A plurality of such user output modes are provided, and the user canselect one of them in consideration of the printing number and the restperiod. As shown in Table 1, in the high speed output mode (second smallsize paper printing mode), the output number per unit time is largerthan in the normal mode, but the continuously outputtable number islimited. Therefore, when the required print number is small, (not morethan 5 or 10 in this embodiment), the selection of the high speed outputmode is advantageous in that the time required for finishing all theprints is relatively shorter. However, in the high speed output mode,the rest of printing time is required after a predetermined number ofprints are continuously outputted, and therefore, when the requiredprint number is large, the selection of the normal mode results in theshorter time until the required number of prints are finished. In thisembodiment, two high speed output modes are prepared in addition to thenormal mode, but the number of the high speed modes may be n (n≧1) forwhich the description of this embodiment similarly applies. In the highspeed output modes, the throughputs may be the same, or may bedifferent, that is, throughput down is used. In the high speed outputmode of 22 ppm, when the small size sheets are continuously outputted,the continuously outputtable number is limited to 5 sheets so that thenon-sheet-passage-part of the fixing portion does not exceed the durabletemperature of the fixing portion. In the high speed output mode of 18ppm, when the small size sheets are continuously outputted, thecontinuously outputtable number is limited to 10 sheets so that thenon-sheet-passage-part of the fixing portion does not exceed the durabletemperature of the fixing portion.

Referring to FIG. 5, the operation of the apparatus of this embodimentwill be described. The processing operation in accordance with the flowchart is carried out by an unshown CPU in the host computer. Here, thesmall size sheet high speed output mode I is the high speed output modenot more than 10 sheet, and the small size sheet high speed output modeII is the high speed output mode not more than 5 sheets.

When the printing instructions for the small size sheets is produced inthe application (step 1 (S1)), the image data is analyzed in step 2, andthe image is generated, and the printing number is calculated. In step3, the discrimination is made as to whether or not small size sheet highspeed output mode I or II is selected by the user, and if so, theoperation goes to step 4, and the selected high speed output mode I orII is transmitted to the image forming apparatus.

If the high speed small size sheet output mode is not selected by theuser, the operation goes to step 5, where the normal small size sheetmode is transmitted to the image forming apparatus.

In this embodiment, it is supposed that the user selects the high speedsmall size sheet output mode for each printing job, but this embodimentis not limited to such an example, and the selection of the high speedsmall size sheet output mode can be registered.

FIG. 7 shows results of measurement and comparison of the temperaturerise of the end portion of the ceramic heater with the settings of thisembodiment. The permissible temperature for the temperature rise of theend of the ceramic heater is 260° C., and the temperature rises in anycase are within the limit.

It has been confirmed empirically that according to this embodiment, 27%speed-up for the case of 5 or less small size sheet outputs and 33%speed-up for 10 or less small size sheet outputs are accomplished.

As described in the foregoing, according to this embodiment, thethroughput of small size sheet printing is improved, when a limitednumber of small size sheets are randomly outputted. This improves thepractical operability. According to this embodiment, it is unnecessaryto change the hardware structure, but modifications in the informationprocessing are required, and therefore the cost for the change is low.

Embodiment 2

The image forming system according to Embodiment 2 will be described. Inthis embodiment, when the user selects a high speed small size sheetoutput mode but there is another mode with which the output speed ishigher as a result of calculation, the mode is automatically switched tothe highest speed output setting. The general structure of thisembodiment is similar to that of Embodiment 1, and therefore, thedetailed description thereof is omitted.

The description will be made as to the case of the high speed outputmodes shown in Table 1.

When the user selects the high speed output mode I for not more than 10,but the actual printing number is not more than 5 as a result of thecalculation in the printer driver, the output mode is automaticallyswitched to the high speed output mode II with which the output speed ishigher.

Referring to the flow chart of FIG. 8, a processing in this embodimentwill be described.

Here, the small size sheet high speed output mode I is the high speedoutput mode for not more than 10 sheets, and the small size sheet highspeed output mode II is the higher high speed output mode for not morethan 5 sheets.

When the printing instructions for small size sheets are produced by theapplication software (step 21), the image data are analyzed to generateimages, and the printing number is calculated in step 22. In step 23,the discrimination is made as to whether or not the high speed smallsize sheet output mode I is selected by the user, and if so, theoperation goes to step 24. If not, the operation goes to step 27 wherethe normal small size sheet mode is transmitted to the image formingapparatus.

The discrimination is made as to whether or not the printing numbercalculated in the step 24 is not more than 5 which is the upper limitnumber in the high speed small size sheet output mode II, and if it isnot more than 5, the operation goes to step 25, and the high speed smallsize sheet output mode II is transmitted to the image forming apparatusas the output mode to be executed. If it exceeds 5, the operation goesto step 26, and the high speed output mode I is transmitted to the imageforming apparatus.

As described in the foregoing, according to this embodiment, when a highspeed output mode, which is higher than the high speed small size sheetoutput mode selected by the user, is applicable, the higher speed modeis automatically applied, and therefore, the operability is improved.

Embodiment 3

The image forming system according to Embodiment 3 will be described. Inthis embodiment, when the calculated printing number is larger than theupper limit number in the high speed small size sheet output modeselected by the user, and there is a high speed small size sheet outputmode applicable to the number, the output mode is automatically switchedto the applicable small size sheet high speed output mode The generalstructure of this embodiment is similar to that of Embodiment 1, andtherefore, the detailed description thereof is omitted.

The description will be made as to the case of the high speed outputmodes shown in Table 1. When the user selects the high speed small sizesheet output mode II for not more than 5 sheets, and the actual printingnumber outputted from the printer driver is more than 5, the mode isautomatically switched to the small size sheet high speed output mode Ifor not more than 10, in this example.

Referring to a flow chart of FIG. 9, the processing in this embodimentwill be described. Here, the small size sheet high speed output mode Iis the high speed output mode for not more than 10 sheets, and the smallsize sheet high speed output mode II is the higher high speed outputmode for not more than 5 sheets.

When the printing instructions for small size sheets are produced by theapplication software (step 31), the image data are analyzed to generateimages, and the printing number is calculated in step 31. In step 33,the discrimination is made as to whether or not the user selects thehigh speed small size sheet output mode II, and if so, the operationgoes to step 34, and if not, the operation goes to step 38 where thenormal small size sheet mode is transmitted to the image formingapparatus.

In step 34, the discrimination is made as to whether or not thecalculated printing number is not more than 5 which is the upper limitnumber in the high speed small size sheet output mode II, and if so, theoperation goes to step 35, and the high speed small size sheet outputmode II is transmitted to the image forming apparatus. If it exceeds 5,the operation goes to step 36, and the discrimination is made as towhether or not the calculated printing number is not more than 10 whichis the upper limit number of the high speed output mode I, and if it isnot more than 10, the operation goes to step 37, where the high speedsmall size sheet output mode I is transmitted to the image formingapparatus. If it exceeds 10, the operation goes to step 38, the normalsmall size sheet mode is transmitted to the image forming apparatus.

As described in the foregoing, according to this embodiment, even if thesmall size sheet high speed output mode required by the user is improperfor the printing number, if a lower speed small size sheet high speedoutput mode is applicable, the applicable small size sheet high speedoutput mode is automatically applied, and therefore, the operability isimproved.

Embodiment 4

The image forming system according to Embodiment 4 will be described. Inthis embodiment, when the calculated printing number is larger than thelimit number of the small size sheet high speed output mode selected bythe user, the output mode is automatically switched to the normal smallsize sheet mode printing The general structure of this embodiment issimilar to that of Embodiment 1, and therefore, the detailed descriptionthereof is omitted. The description will be made as to the case of thehigh speed output modes shown in Table 1. When the user selects the highspeed output mode for not more than 5, and the actual printing numberoutputted from the printer driver is larger than 6, the operationautomatically goes out of the high speed output mode to the normal smallsize sheet mode printing.

FIG. 10 shows a flow chart of the data processing. When the printinginstructions for small size sheets are produced by the applicationsoftware (step 41), the image data are analyzed to generate images, andthe printing number is calculated in step 42. In step 43, thediscrimination is made as to whether or not small size sheet high speedoutput mode I or II is selected by the user, and if so, the operationgoes to step 44, and if not, it goes to step 46, where the normal smallsize sheet mode is transmitted to the image forming apparatus In step44, the discrimination is made as to whether or not the calculatedprinting number is not more than the upper limit number of the selectedsmall size sheet high speed output mode. If it is not more than theupper limit number, the operation goes to step 45, where the high speedsmall size sheet output mode is transmitted to the image formingapparatus If it exceeds the upper limit number, the operation goes tostep 46, and the normal small size sheet mode is transmitted to theimage forming apparatus irrespective of selection of the small sizesheet high speed output mode.

As described in the foregoing, according to this embodiment, the mode isdetermined taking into account the calculated printing number, andtherefore, the operation more assured than in Embodiment 1.

Embodiment 5

The image forming system according to Embodiment 5 will be described. Inthis embodiment, when the small size sheet high speed output mode isselected, the availability of the high speed small size sheet outputmode is determined on the basis of an initial detected temperature ofthe temperature detecting element of the heat-fixing device, in thisexample. The general structure of this embodiment is similar to that ofEmbodiment 1, and therefore, the detailed description thereof isomitted.

In this embodiment, when the initial detected temperature of thetemperature detecting element disposed on a back side of the heatersubstrate of the heating fixing device is not more than 100° C., theexecution of the small size sheet high speed output mode printing ispermitted When it is higher than 100° C., the mode is automaticallyswitched to the normal mode printing even if the small size sheet highspeed output mode printing is selected.

FIG. 11 shows a flow chart of the data processing. When the printinginstructions for small size sheets are produced by the applicationsoftware (step 51), the image data are analyzed to generate images, andthe printing number is calculated in step 52. In step 53, thediscrimination is made as to whether or not the small size sheet highspeed output mode I or II is selected by the user, and if so, theoperation goes to step 54 If not, the operation goes to step 56, thenormal small size sheet mode is transmitted to the image formingapparatus. In step 54, the discrimination is made as to whether or notthe initial detected temperature of the temperature detecting element isnot more than 100° C., and if it is not more than that, the selectedsmall size sheet high speed output mode is transmitted to the imageforming apparatus, when the temperature exceeds 100° C., the operationgoes to step 56, and the normal small size sheet mode is transmitted tothe image forming apparatus. By doing so, the damage of the fixingdevice attributable to the over-heating can be prevented

Embodiment 6

The image forming system according to Embodiment 6 will be described. InEmbodiments 2-5, selection of the printing mode is carries out in thehost computer, but in this embodiment, the selection of the printingmode is carries out in the image forming apparatus upon the small sizepaper printing Operates are similar to those of Embodiments 1-6, but thesetting which is considered as being optimum as the printing performancefor the small size sheet is built in beforehand, by which the user isnot required to carry out an additional setting on the host computer301, thus facilitating the operation. FIG. 12 is a flow chart the dataprocessing in this embodiment

In this embodiment, when the host computer produces the printinginstructions for the small size sheet (step 61), the print data areanalyzed, and the image formation and determining of the printing numberare carried out (step 62), and thereafter the printing information issent to the image forming apparatus (step 63). The image formingapparatus discriminates on the basis of the received information whetheror not the print is on a small size sheet, and if it is on the smallsize sheet, it is checked whether or not the initial temperature of thetemperature detecting element provided on the back side of the heatersubstrate of the heating fixing device is not more than the threshold(100° C., here), to discriminate whether or not the small size sheethigh speed output mode is applicable (step 65) When the small size sheethigh speed output mode is applicable, and the number of the printing jobis not more than 5 as a result of referring to the print job number(step 66), the small size sheet high speed output mode I is used (step67). In the small size sheet high speed output mode I, the prints areoutputted at the full speed 22 ppm, and thereafter, 10 sec rest time isexecuted. If the number in the printing job is not less than 6 and notmore than 10, the small size sheet high speed output mode II is applied(step 68) In the small size sheet high speed output mode I, the printsare outputted at the full speed 18 ppm, and thereafter, 15 sec rest timeis executed. If the number in the printing job is not less than 11, thenormal small size sheet mode is used in which the throughput speed isstepwisely decreased in accordance with the printing number (step 69).In this embodiment, the speed is 18 ppm up to 3 sheets, 14 ppm up to 6sheets, 9 ppm up to 11 sheets, and 7 ppm up to 21 sheets, and 6 ppm for22 and more sheets.

The settings are determined in consideration of the frequency of thecontinuous printing numbers of small size sheet, the frequency of theintervals of the generations of the printing job, and the sensoryconvenience. The embodiment shows only an example, and the applicabilitytemperature threshold setting, the printing number of the small sizesheet, the throughput, and the rest time are not limited to theforegoing examples.

Embodiment 7

Embodiment 7 of the present invention will be described. This embodimentis different from Embodiments 1-6 in that the recording material feedingspeed in the second small size paper printing mode (high speed smallsize sheet output mode) is higher than the first small size paperprinting mode (small size sheet normal output mode). The targettemperature (fixing temperature) during the fixing process of the fixingportion in the first small size paper printing mode is set to be lowerthan the target temperature (fixing temperature) during the fixingprocess of the fixing portion in the second small size paper printingmode.

[Image Forming Apparatus]

Part (a) of the FIG. 13 is a schematic illustration of a color imageforming apparatus according to Embodiment 7, wherein the image formingapparatus of the this embodiment is a tandem type full color printerusing an electrophotographic system, in which recording materials up toA3 size can be processed. The image forming apparatus comprises fourimage forming station (image forming units), namely, image formingstations 1Y, 1M, 1C, 1Bk for forming yellow (Y), magenta (M), cyan (C)and black (Bk) images, respectively, and they are arranged in one lineat constant intervals In the Figure, a, b, c and d correspond to Y, M, Cand Bk, respectively, and are omitted unless they are necessary.

When the start signal for the image forming operation is produced, thephotosensitive drum 2 of the image forming station 1 is rotated in adirection indicated by the arrow at a predetermined process speed(peripheral speed), and is charged uniformly to a negative polarity, forexample. An exposure device 7 converts the image signal inputted andcolor-separated to a light signal by a laser output portion (unshown),and the laser beam as the light signal scans the charged photosensitivedrum 2 to form an electrostatic latent image. The developing device 4 ais supplied with a developing bias voltage having the same polarity asthe charge polarity (negative) to electrostatically deposit yellow toneronto the electrostatic latent image formed on the photosensitive drum 2a in accordance with the charged potential, thus visualizing theelectrostatic latent image into a developed image. The transfer roller 5a is supplied with a primary transfer bias having a polarity oppositethat of the toner (positive) to transfer (primary transfer) the yellowtoner image onto an intermediary transfer belt 40 rotated in thedirection indicated by the arrow by a driving roller 141 in a primarytransfer nip N, and the intermediary transfer belt 40 advances towardthe image forming station 1M. In the same manner, on the yellow tonerimage on the intermediary transfer belt 40, magenta, cyan and blacktoner images formed on the photosensitive drums 2 b, 2 c, 2 d aresequentially overlaid in the primary transfer portions N, thus forming afull-color toner image.

A registration roller 146 feeds the recording material P to a secondarytransfer nip M in timed relation with movement of the leading end of thefull-color toner image on the intermediary transfer belt 40 to thesecondary transfer nip M. The secondary transfer roller 144 is suppliedwith a secondary transfer bias voltage having a polarity opposite thatof the toner (positive) to transfer the full-color toner image alltogether onto the recording material (secondary transfer). a fixingdevice 12 heats and presses the fed recording material P by the fixingnip between the fixing sleeve 20 and the pressing roller (pressingmember) to fuse and fix the toner image on the recording material PThereafter, the recording material P is discharged to the outside, bywhich the series of image forming operations is completed. Theuntransferred toner remaining on the photosensitive drum 2 during theprimary transfer is removed and collected by a drum cleaning device 6,and the after-secondary-transfer residual toner remaining on theintermediary transfer belt 40 after the secondary transfer is removedand collected by a belt cleaning device 145.

The image forming apparatus includes a ambient condition sensor 37 to beused for adjustment of the density of the toner image formed on therecording material P and for accomplishing optimum transfer and fixingconditions the conditions of the bias voltages of the charging, thedevelopment, the primary transfer and the secondary transfer can bechanged in accordance with the ambient condition (temperature andhumidity) in the image forming apparatus In order to accomplish theoptimum transfer and fixing conditions for the recording material P, amedia sensor 38 is provided, and the kind of the recording material P isdiscriminated to change the transfer bias and the fixing condition

[Fixing Device 12]

Part (b) of the FIG. 13 is a schematic illustration of the fixing device12 of the this embodiment, and the fixing device 12 is a heatingapparatus of fixing sleeve heating type and pressing rotating memberdrive type (tensionless type) The fixing sleeve 20 is a cylindrical(endless belt) member comprising a belt and a elastic layer thereon, andthe pressing roller 22 is a back-up member, and a heater holder 17 is aheat resistive rigid member having a substantially arcuate cross-section(trough like). The fixing heater 16 is a heating element (heat source)and is a ceramic heater, for example, and is extended along thelongitudinal direction (perpendicular in the feeding direction of therecording material) of the heater holder 17 on the lower surface of theheater holder 17. The fixing sleeve 20 is loosely telescoped around theheater holder 17. The heater holder 17 is made of liquid crystal polymerresin material having a high heat resistive and supports the fixingheater 16 and guides the fixing sleeve 20 In this embodiment, it isliquid crystal polymer (Sumicus Super LCP, E4205L (tradename theavailable from Sumitomo Kagaku Kabushiki Kaisha, Japan). The maximumusable temperature of E4205L (limit temperature due to flexure by theload) is approx. 305° C.

The pressing roller 22 comprises a hollow core metal of aluminum, steel(STKM, carbon steel tube for machine structure JIS G 3445) or the like,a silicone rubber layer having a thickness of approx. 3 mm thereon, anda PFA resin material tube having a thickness of approx. 50 μm thereon.The opposite end portions of the pressing roller 22 are rotatablysupported by bearings provided at the rear side and the front side ofthe device frame 24 Above the pressing roller 22, a fixing sleeve unitincluding the fixing heater 16, the heater holder 17, the fixing sleeve20 and so on is provided in parallel with the pressing roller 22, withthe fixing heater 16 facing down. The opposite end portions of theheater holder 17 are urged toward the axis of the pressing roller 22 byan unshown pressing mechanism by a force of 147 N (15 kgf) at each end,and total pressure of 294 N (30 kgf). By doing so, the downward surfaceof the fixing heater 16 is urged toward the elastic layer of thepressing roller 22 through the fixing sleeve 20 against the elasticityof the elastic layer at a predetermined urging force to form a fixingnip 27 having a predetermined width enough for the heating and fixing.The pressing mechanism is provided with an automatic pressure varyingmechanism to change the pressure in accordance with the kind of therecording material P.

Designated by 23 and 26 are an entrance guide and a fixing and sheetdischarging roller, and the entrance guide 23 guides the recordingmaterial P such that the recording material P fed from the secondarytransfer nip M is correctly guided to the fixing nip 27. In thisembodiment, the entrance guide 23 is made of Hyperlite (tradename) whichis a reformulated PET (polyethylene terephthalate) resin materialavailable from Kabushiki Kaisha Kaneka, Japan.

The pressing roller 22 is rotated at a predetermined peripheral speed inthe counterclockwise direction indicated by arrow by unshown drivingmeans, and the rotational force is applied to the fixing sleeve 20 bythe press-contact frictional force in the fixing nip 27. The fixingsleeve 20 is rotated in the clockwise direction indicated by the arrowoutside the heater holder 17, while the inner surface of the fixingsleeve 20 is in sliding close-contacted with the downward surface of thefixing heater 16. Grease is applied to the inner surface of the fixingsleeve 20 to assure the slidability between the heater holder 17 and theinner surface of the fixing sleeve 20. The pressing roller 22 isrotated, and the fixing sleeve 20 is rotated thereby, and the fixingheater 16 is supplied with the electric power to heat it to apredetermined temperature, and is controlled in the temperature by thecontroller 21. In such a state, the recording material P carrying theunfixed toner image t is introduced along the entrance guide 23 into thefixing nip 27. By the fixing nip 27, the recording material P is nippeda fed while the toner image carrying side of the recording material P isin contact with the outer surface of the fixing sleeve 20. The heat ofthe fixing heater 16 is applied to the recording material P through thefixing sleeve 20, and the unfixed toner image on the recording materialP is heated and pressed so that it is fused and fixed. The recordingmaterial P having passed through the fixing nip 27 is separated by thecurvature from the fixing sleeve 20 and is discharged by the fixing andsheet discharging roller 26.

[Fixing Heater 16]

Part (a) of FIG. 14 is a sectional view of the fixing heater 16. Thealumina substrate 41 is a ceramic substrate elongated in the directionperpendicular to the feeding direction of the recording material P. Theheat generating resistor layers 42, 43 (43 a, 43 b) (electric heatgenerating resistance layer) (heat generating element) are heatingelements each having a thickness of approx. 10 μm and a width of 1 mm,painted in the form of line or band extending in the longitudinaldirection by screen printing. For the heat generating elements 42, 43,an electroconductive paste including silver-palladium (Ag/Pd) alloywhich generates heat by current therethrough is printed on the aluminasubstrate 41. For the electrode portion 44 ((b) of FIG. 2), a silverpaste is printed by screen printing or the like into a pattern on afront side of the alumina substrate 41 as an electric power supplypattern for the heat generating elements 42, 43. A glass coating 45having a small thickness of approx. 60 μm is provided to protect theheat generating elements 42, 43 to assure the insulativeness. Thesliding layer 46 of polyimide is provided on the side of the aluminasubstrate 41 contacting the fixing sleeve 20.

Part (b-1) of FIG. 14 shows a front side of the fixing heater 16, andpart (b-2) FIG. 14 shows a heat generation distribution of the fixingheater 16. The heat generating element 42 has a resistance ratio, perunit length, of the end to the central portion with respect to thelongitudinal direction of the heater, which is larger than that of theheat generating element 43. The heat generating element 43 (43 a, 43 b)continuously increases in its width from the longitudinally centerportion, and therefore, the amount of heat generation graduallydecreases from the longitudinally central region toward the end portion.On the other hand, the heat generating element 42 continuously decreasesin its width from the longitudinally center portion toward the endportion, and therefore, the amount of heat generation graduallyincreases from the longitudinally central region toward the end portion.Thus, the amount of heat generation is changed continuously in thelongitudinal direction so that the non-sheet-passage-part temperaturerise (end portion temperature rise) can be effectively suppressed in afixing device applicable for a wide variety of sheet size up to A4 size.The electrode portion 44 of the fixing heater 16 is provided with aelectric energy supply connector, and the electric energy supply iseffected to the electrode portion 44 through the electric energy supplyconnector from the heater driving circuit portion, by which the heatgenerating elements 42, 43 generates heat to quickly raise thetemperature of the fixing heater 16. In the normal use, the rotation ofthe fixing sleeve 20 starts with the start of rotation of the pressingroller 22, so that with rise of the temperature of the fixing heater 16,the inner surface temperature of the fixing sleeve 20 rises. Thecontroller 21 controls the electric power supply to the fixing heater 16by a PID control so that the detected temperature of the sleevethermister 18 ((b) of the FIG. 13) indicative of the inner surfacetemperature of the fixing sleeve 20 is the target value.

Part (c) of the FIG. 14 shows a positional relation between the fixingheater 16 and the thermister. In this embodiment, in order to detect thenon-sheet-passage-part temperature rise at the time of the recordingmaterial having a width smaller than the maximum processable width isbeing fed, end thermistors 28 are provided at the opposite ends inaddition to the sleeve thermister 18 and the main thermister 19. Here,the width of the recording material is a dimension of the recordingmaterial measured in the direction perpendicular to the feedingdirection of the recording material. The sleeve thermister 18 fordetecting the inner surface temperature of the fixing sleeve 20 isprovided with a thermister element mounted to the free end of the arm 25of stainless steel fixed to the heater holder 17 ((b) of the FIG. 13).By the elastic swing of the arm 25, the contact of the thermisterelement to the inner surface of the fixing sleeve 20 can be alwaysassured even when the movement of the inner surface of the fixing sleeve20 is unstable. The main thermister 19 contacts the neighborhood of thelongitudinally center portion of the fixing heater 16 to detect thetemperature of the back side of the fixing heater. The end thermister 28is provided in the non-sheet-passage-part range with respect to the LTRsize (landscape, width of 279 mm), so that the non-sheet-passage-parttemperature can be detected at the time of the LTR size recordingmaterial being fed. In this embodiment, the controller 21 controls theelectric power supply to the fixing heater 16 so that the detectedtemperature of the main thermister 19 maintains the set temperature, butwhen the detected temperature of the sleeve thermister 18 deviates fromthe target value, the set temperature to be compared with the detectedtemperature of the main thermister 19 is corrected.

[Fixing Sleeve 20]

In this embodiment, the fixing sleeve 20 comprises a cylindrical endlessbelt (belt base material) of SUS having a thickness of 30 μm, and asilicone rubber layer (elastic layer) having a thickness of approx. 300μm. On the silicone rubber layer, a PFA resin material tube (outermostlayer) having a thickness of 20 μm is provided. The thermal capacity ofthe fixing sleeve 20 was measured as 2.9×10-2 cal/cm²·° C. per 1 cm² offixing sleeve. The base layer of the fixing sleeve 20 may be ofpolyimide or the like, but SUS is better than polyimide in that thethermal conductivity is approx. 10 times, and therefore, the on-demandproperty is better. For the elastic layer of the fixing sleeve 20, arubber layer exhibiting a high thermal conductivity is used in order toprovide a high on-demand property, and the specific heat thereof is2.9×10-1 cal/g·° C. On the surface of the fixing sleeve 20, afluorinated resin material layer is provided, by which the partingproperty of the surface is improved, and the offset phenomenon—whichresults from the toner being deposited once onto the surface of thefixing sleeve 20 and then moving to the recording material P again canbe prevented. Because the fluorinated resin material layer at thesurface of the fixing sleeve 20 is in the form of a PFA tube, thefluorinated resin material layer can be easily made uniform.

Generally, with the increase of thermal capacity of the fixing sleeve20, the temperature rising becomes dull with the result of deteriorationof the on-demand property. For example, when it is supposed that in adevice in which the heater is at rest during the stand-by period, andthe temperature rises sufficiently within 1 minute from the printinstructions without temperature control, it is necessary that thethermal capacity of the fixing sleeve 20 has to be not more than 1.0cal/cm²·° C. In this embodiment, the device is designed such that in thecase that the voltage source is actuated a while after deactuation ofthe voltage source, the temperature of the fixing sleeve 20 issufficiently heated up to 190° C. within 20 seconds from actuation ofthe electric power supply of 1000 W to the fixing heater 16. When thespecific heat of the silicone rubber layer is approx. 2.9×10-1 cal/g·°C., the of the silicone rubber has to be not more than 500 μm, and it isnecessary that the thermal capacity of the fixing sleeve 20 has to benot more than approx. 4.5×10-2 cal/cm²·° C. On the contrary, if it isnot more than 1.0×10-2 cal/cm²·° C., the rubber layer of the fixingsleeve 20 is extremely thin, and the image quality such as OHTtransparency and/or glossiness evenness results in being equivalent tothat of an on-demand fixing device not provided with an elastic layer.

In this embodiment, the thickness of the silicone rubber necessary toprovide a high image quality image of satisfactory OHT transparency andglossiness is not less than 200 μm, and in such a case, the thermalcapacity is 2.1×10-2 cal/cm²·° C. That is, generally, the thermalcapacity of the fixing sleeve 20 is not less than 1.0×10-2 cal/cm²·° C.and not more than 1.0 cal/cm²·° C. In this range, in order to accomplishboth of the on-demand property and the high image quality, the fixingsleeve of this embodiment is in range not less than 2.1×10-2 cal/cm²·°C. and not more than 4.5×10-2 cal/cm²·° C.

[Control of Throughput in this Embodiment]

The image forming apparatus of this embodiment is operable with twoimage forming speeds. The first image forming speed for a second smallsize paper printing mode (high speed small size sheet output mode) isapprox. 150 mm/sec, and a second image forming speed for the first smallsize paper printing mode (normal small size sheet output mode) is lowerthan the first image forming speed, and is approx. ⅔ of that speed whichis approx. 100 mm/sec. That is, in the second small size paper printingmode, the recording material feeding speed in the heating fixing portionis higher than that in the small size paper printing mode.

Part (a) of FIG. 15 is a graph showing a relation between the continuousprint number and the throughput (ppm: number of prints per one minute),when small size sheets are fed under the low temperature ambientcondition (approx. 15° C.) at the first image forming speed and thesecond image forming speed. The used small size sheet is BusinessMultipurpose white paper 4200 available from Xerox Corporation and has aletter-size (width of 216 mm×length of 279.4 mm) and a basis weight ofapprox. 90 g/m².

The fixing temperature in the case of first image forming speed(detected temperature of the sleeve thermister 18) is approx. 175° C.from the standpoint of the fixing property. When the sheet is passed atthe first image forming speed, the speed is 20 ppm at the initial stage,and when approx. 15 sheets are processed, the detected temperature ofthe end thermister 28 reaches a throughput down threshold temperature(approx. 270° C., for example) due to the non-sheet-passage-parttemperature rise. Then, the throughput is lowered from the 20 ppm downto 10 ppm (the image forming speed remains unchanged, that is, 150mm/sec, but the sheet interval is expanded). Thereafter, the detectedtemperature of the end thermister 28 reaches the throughput downthreshold again at approx. 150 sheets processed, and the throughput islowered to 8 ppm from 10 ppm (the image forming speed remains unchanged,that is, 150 mm/sec, but the sheet interval is further expanded).Thereafter, the detected temperature of the end thermister 28 reachesthe throughput down threshold again at approx. 193 sheets processed, andthe throughput is lowered to 6 ppm from 8 ppm (the image forming speedremains unchanged, that is, 150 mm/sec, but the sheet interval isfurther expanded). As will be understood, when the image forming speed(fixing process speed) is fixed at the first image forming speed, thethroughput (output number per unit time) gradually lowers if the printnumber is large.

The fixing temperature in the second image forming speed operation isapprox. 155° C. which is lower than the fixing temperature setting inthe first image forming speed operation since the image forming speed islower than the first image forming speed. Therefore, the fixing speed isslow, and the fixing temperature per se is low, and therefore, thenon-sheet-passage-part temperature rise is low, and when the sheet isfed at the second image forming speed, the initial speed is approx. 13.4ppm, and thereafter, the end thermister 28 does not reach the throughputdown threshold temperature.

In view of this, in this embodiment, if the required print number in thesmall size sheet print is not more than a predetermined number(continuously outputtable number), the mode is set to the second smallsize paper printing mode (image forming speed is fixed at the firstspeed), and the printing is executed, and when the print number islarger than the predetermined number, the mode is set to the first smallsize paper printing mode the image forming speed is fixed to the secondspeed), and the printing is executed.

FIG. 17 is a flow chart of throughput control in a comparison example.In the comparison example, when the print instructions is produced instep 1001 (S1001 or the like) and if the operation is not for small sizesheet (S1002), the printing is executed at the first image forming speed(S1004). If the operation is for small size sheet (S1002), the printingis executed at the second image forming speed which is lower than thefirst image forming speed In the comparison example, the image formingspeed is fixed corresponding to the passing paper size Part (b) of theFIG. 15 shows a average throughput at the time of small size sheetprocessing in this example as comparison example 1. In comparisonexample 1 wherein the speed is fixed to the second image forming speed,the initial average throughput is approx. 13.4 ppm, and the averagethroughput is approx. 13.4 ppm even if the print number is large.

Another comparison example was checked. Part (b) of the FIG. 15 is agraph of comparison example 2 in which the image forming speed for smallsize sheet processing is fixed to the first image forming speed, and thenon-sheet-passage-part temperature rise is prevented by expanding thesheet intervals. In comparison example 2, the throughput is high, thatis 20 ppm in the initial stage, but the when approx. 14 sheets areprocessed, the throughput lowers due to the non-sheet-passage-parttemperature rise (the sheet intervals are expanded). Therefore, theaverage throughput lowers with increase of the number of prints.

FIG. 16 is a flow chart showing the throughput control in thisembodiment. When the print instructions is produced in step S101, andthe unshown engine controller discriminates that the sheet is not asmall size sheet in step S102, the printing is executed at the firstimage forming speed in step S107, as is the same with the foregoingcomparison example. If the engine controller discriminates in step S102that the sheet size is smaller than a predetermined width, that is B5,A5, EXE, A4 longitudinal for example, the operation goes to S103. Instep S103, the engine controller checks the print JOB number (number ofimage formations), and compares it with a predetermined image formingspeed switching number in step S104. If, in step S104, the enginecontroller discriminates that the print JOB number is smaller than thepredetermined number, that is, the image forming speed switching number,and the control is executed for the printing at the first image formingspeed, in step S105. If, in step S104, the engine controllerdiscriminates that the print JOB number is larger than the predeterminednumber, that is, the image forming speed switching number, the controlis executed for the printing at the second image forming speed which islower than the first image forming speed in step S106. The image formingspeed switching number (predetermined number) is 30, for example.

Parts (b) and (c) of the FIG. 15 show the print JOB number and theaverage throughput at comparison examples 1, 2. By this embodiment, whenthe print JOB number is smaller than the image forming speed switchingnumber (14 sheets), the printing is completed at 20 ppm, and therefore,the average throughput (average ppm) is larger than in comparisonexample 1. When the print JOB number is 15-30, the speed of 20 ppm withthe speed 150 mm/sec is maintained in the period of printing the first14 sheets. In the period of 15th to 30th sheets, the speed of 150 mm/secis maintained, and the sheet intervals are expanded, and therefore, theoutput speed is 10 ppm, but the average throughput from the first to theend (not more than 30) is not less than 13.4 ppm. By this embodiment,the print JOB number is larger (100, 200) than the image forming speedswitching number (30), the image forming speed is 100 mm/sec from thefirst print, and the fixing temperature is lower than in the case of 150mm/sec of the image forming speed, and therefore, it is not necessary toexpand the sheet intervals significantly, the average throughput fromthe first to the end is 13.4 ppm, and the average throughput (averageppm) is larger than in the comparison example 2.

As described in the foregoing, according to this embodiment, the imageforming speed is switched in accordance with the number of print jobs,and therefore, the productivity (performance) in the case of small sizesheet processing can be improved, and the lifetimes of the image formingstation and the fixing device or the like can be expanded.

INDUSTRIAL APPLICABILITY

According to the present invention, when limited numbers of small sizesheets are outputted at times, the throughput can be increased. Thisimproves the practical operability. According to the present invention,it is unnecessary to change the hardware structure, and the informationprocessing software change is enough, and therefore, the required costis low.

The invention claimed is:
 1. An image forming apparatus comprising: animage forming portion for forming a toner image on a recording material;and a heating fixing portion for heating and fixing the toner image onthe recording material, wherein for a printing on a small size sheethaving a width smaller than a maximum processable width of said imageforming apparatus, said apparatus is operable in a first small sizesheet printing mode, in which a number of continuously printable sheetsis not limited, and a second small size sheet printing mode in which anumber of continuously printable sheets is limited and in which anoutput number per unit time is greater than that in the first small sizesheet printing mode, wherein when a new printing command is inputtedduring execution of a current printing job in the first small size sheetprinting mode, a new printing job is started after completion of thecurrent printing job without a rest period, and wherein when a newprinting command is inputted during execution of the current printingjob in the second small size sheet printing mode, a new printing job isstarted after the completion of the current printing job, with apredetermined rest period between the completion of the current printingjob and the start of the new printing job.
 2. An apparatus according toclaim 1, wherein in the second small size sheet printing mode, arecording material feeding speed in said heating fixing portion ishigher than that in the first small size sheet printing mode.
 3. Anapparatus according to claim 2, wherein in the first small size sheetprinting mode, a target temperature in a fixing process of said heatingfixing portion is lower than that in the second small size sheetprinting mode.
 4. An apparatus according to claim 1, wherein saidheating fixing portion includes a fixing film, a heater contacting aninner surface of said fixing film, and a pressing roller forming afixing nip together with said heater through said fixing film.
 5. Anapparatus according to claim 1, wherein the first small size sheetprinting mode and the second small size sheet printing mode are userselectable.
 6. An apparatus according to claim 1, wherein selecting amode from the first small size sheet printing mode and the second smallsize sheet printing mode is carried out in said image forming apparatusin accordance with a required print number.
 7. An apparatus according toclaim 1, wherein the first small size sheet printing mode is used withthe output number per unit time being stepwisely decreased in accordancewith a printing number.